1. Field of the Invention
The present invention pertains to towed-array marine seismic surveys, and, more particularly, to towed-array marine seismic surveys employing generally advancing curved sail lines.
2. Description of the Related Art
This section of this document is intended to introduce various aspects of the art that may be related to various aspects of the present invention described and/or claimed below. This section provides background information to facilitate a better understanding of the various aspects of the present invention. As the section's title implies, this is a discussion of related art. That such art is related in no way implies that it is also prior art. The related art may or may not be prior art. It should therefore be understood that the statements in this section of this document are to be read in this light, and not as admissions of prior art.
The exercise of examining subterranean geological formations for deposits of hydrocarbon deposits is known as “seismic surveying”. Sometimes the geological formations lie beneath a body of water. This type of seismic survey is known as a “marine” seismic survey. Marine seismic surveys may be performed in salt, fresh, or brackish waters and are not limited to saltwater environments.
One type of marine seismic survey is called a “towed-array” seismic survey. In such a survey, a tow vessel tows an array of equipment along a straight sail line. The array usually includes a number of seismic streamers, typically up to eight of them, up to several kilometers long. The streamers are populated with a number of instruments, most notably seismic receivers such as hydrophones. Sometimes the array will also include a plurality of seismic sources. As the array is towed, the streamers ideally straighten and roughly parallel each other. Sometimes environmental or survey conditions adversely impact the shape of the array, but this is the ideal shape.
Thus, in conventional seismic survey acquisition, it is typical to determine where the components of equipment deployed in the sea to acquire seismic data are in relation to the sail line. These lines are straight with the exception of special acquisition patterns such as undershoot obstruction avoidance. Another exception is 4D acquisition that has an objective to follow a path that was traversed during an earlier survey.
All these acquisition patterns can be examined with respect to the planned path with a conventional straight line reference. This is often described as “along” and “across” or “cross line” and “in line”. Distance from the planned pre-plot line to the shot event position of the seismic equipment is determined for the purpose of quality control in this reference frame. In addition to quality control, steering equipment such as streamer and source steering devices and systems and the towing vessel itself can make decisions based on variations in the conventional reference frame. The reference frame used has the same origo and the axes the same direction for at least one shot. In the case of the vessel reference frame the vessel motion is irrelevant since the vessel structure rigid body reference system is the reference. In the case of a survey area reference frame the vessel and towed equipment are all evaluated with respect to an origo and orthogonal axes fixed with respect to the earth.
Traditionally seismic surveys have been shot along straight lines defined by a start and end point defining a sail line. A lot of quality control (“QC”) and statistics to users are simply based on a coordinate system with origo in the first point and Y axis in the direction of the second point. Two central numbers used are the “distance across” (or, “DC”) and the “distance along”, or (“DA”).
The distance along is the y coordinate in this coordinate system, and distance across is the x coordinate. The direction from the first to the second point is called the line direction. The sources are usually fired evenly along this line—for example, every 25 m. The point where the source fires is called a “shot point,” and each shot point is given a shot point number. The shot point number will decrease or increase along the line.
The survey of some complex subterranean structures benefits from particular types of towed-array surveys sometimes called “wide-azimuth”, “rich-azimuth”, or multi-azimuth surveys. However, these types of surveys are typically expensive both in terms of survey time and survey resources. One recently developed approach to address these concerns is what is known as a “coil shoot”. A coil shoot differs from a traditional shoot significantly in that the path is intended to be curved rather than straight. With coil shooting a seismic ‘line’ can consist of any sequence of circular segments and straight segments. One difficulty associated with coil shoots is that a lot of the existing statistics and attributes calculated make little sense with coil shooting. With coil shooting a seismic ‘line’ can consist of any sequence of circular segments and straight segments. The quality control techniques used in conventional surveys with respect to the plan are not meaningful when acquiring a curved acquisition pattern.
The present invention is directed to resolving, or at least reducing, one or all of the problems mentioned above.